Signed-off-by: Michael Bridgen <[email protected]>

This adapts the controller so that it will honour the
`.spec.push.branch` field.

The behaviour _without_ that field is to check out the branch given in
`.spec.checkout.branch`, commit, and push to the origin.

With `.spec.push.branch` present, it will try to check out that
branch; if it doesn't exist, it'll create it, starting from
`.spec.checkout.branch`. Either way it'll commit to that branch and
push to the origin.

The effect is that all automation will happen on the "push" branch,
and (most likely) not be applied into the cluster until merged into
whichever branch is synced. When the push branch is deleted, it'll be
created anew; otherwise, commits will pile up there as more changes
are made.

Signed-off-by: Michael Bridgen <[email protected]>